Field sensitive transducers, such as magnetoresistive transducers have several properties that make them desirable read heads for magnetic recording systems. Since their output voltage signal is proportional to the input current signal, magnetoresistive transducers can produce large magnitude output signals. When shielded from unwanted magnetic signals, these transducers can read high density recordings. Unlike inductive read heads, which detect changes in magnetic flux; magnetoresistive transducers detect magnetic flux. This feature makes them desirable in low velocity recording systems.
However, magnetoresistive transducers have three problems. First, small magnetoresistive transducers, desirable for high density data applications, often exhibit Barkhausen noise. Barkhausen noise originates from changes in the transducer's magnetic domain structure. Schemes to suppress magnetic domain activity in these transducers is outlined in the prior art. The article Magnetics of Small Magnetoresistive Sensors written by Ching Tsang and published in The Journal Of Applied Physics of Mar. 15, 1984 describes schemes to suppress domain activity and which is incorporated by reference herein. Secondly, magnetoresistive transducers exhibit the nonlinear characteristic shown in FIG. 5. FIG. 5 plots the output voltage of magnetoresistive head versus the magnetic field applied to the magnetoresistive head. The Tsang article incorporated above lists several biasing schemes that force magnetoresistive heads to operate in its linear range.
In order to take advantage of magnetoresistive head's ability to read high density data, the head needs insulation from magnetic fields emitted by surrounding devices. Shields located on either side of the head, as shown in FIGS. 1A and 1B accomplish this. However, these shields have four major problems. First, the shields may obtain an undesirable magnetic domain structure that exposes the magnetoresistive transducer to several magnetic domains as shown in FIGS. 3A and 3B. The boundaries of the magnetic domains produce abrupt changes in magnetic fields that distorts the transducer's output. Secondly, the shields may amplify and focus the stray magnetic fields emitted by surrounding devices commonly found in disc drives such as motors. The amplified and focused stray magnetic field may affect the magnetoresistive transducer's operation, change the transducer's biasing so that it operates in the non-linear range shown in FIG. 5, or erase the information stored on magnetic storage unit. Thirdly, shields with high magnetic reluctance or poor high frequency response may limit the speed at which the magnetoresistive transducer can read data.